1. Field of the Invention
This invention relates generally to direct access storage devices and, more particularly, to detection of errors in the readback signal of such devices.
2. Description of the Related Art
In a conventional computer data storage system having a rotating storage medium, such as a magnetic or magneto-optical disk, data is stored in a series of concentric or spiral tracks across the surface of the disk. A magnetic disk, for example, can comprise a disk substrate having a surface on which a magnetic material is deposited. The digital data stored on a disk comprises magnetic information that is represented as a series of variations in magnetic orientation of the disk magnetic material. The variations in magnetic orientation, generally comprising reversals of magnetic flux, represent binary digits of ones and zeroes that in turn represent data. The binary digits must be read from and recorded onto the disk surface. A read/write transducing head produces and detects variations in magnetic orientation of the magnetic material as the disk rotates relative to the head.
A computer data storage system may include multiple magnetic disks. The tracks of each disk may be partitioned by sectors having a short servo track information area followed by a user data area. The servo track information area typically includes a sector marker, track identification data, and a servo burst pattern, which are recorded at the time of disk manufacture. The sector marker indicates to the read/write transducing head that servo information immediately follows in the track. The user data area contains data recorded by an end user, or disk drive customer. The head used for reading servo data is typically the same head used for reading customer data.
A magneto-resistive (MR) transducing head includes an MR element that exhibits a change in resistance when in the presence of a changing magnetic field. The change in resistance of the MR element is transformed into a voltage signal by passing a constant bias current through the MR element. For a given MR head, the value of the voltage is the product of the constant bias current and the total resistance between the lead terminals of the head. In this way, the MR head generates a fluctuating voltage readback signal as the MR head is passed over the magnetic information recorded on the disk magnetic material. In a direct access data storage system using digital demodulation, the fluctuating readback signal is digitized and the digital data values of the sampled readback signal are processed to recover the recorded data.
A transient disturbance in the MR head readback signal can result from a thermal asperity, which occurs when a hard particle on the surface of the disk collides with the MR head. The collision causes a rapid temperature rise in the MR element that can be greater than 100 degrees Centigrade (100xc2x0 C). The temperature increase first occurs at the point of contact between the particle and the MR element. The localized temperature increase produces a small but sudden increase in temperature of the entire MR head within 50 to 100 nanoseconds. The MR element has a temperature coefficient of resistance equal to approximately 0.02% per degree C, such that the resistance of the head can increase by several percentage points. This can result in a dramatic change in the voltage of the readback signal.
An MR head has voltage non-linearities that increase with increasing readback signal variations relative to the sensor bias current. As a result, MR heads are designed to maintain signal variations sufficiently small to ensure usable sensor linearity. Voltage variations due to thermal asperities can be many times the typical base-to-peak signal variation produced by the MR head. In addition to the high amplitude change in the readback signal, a thermal asperity also exhibits a relatively long decay time. The high amplitude change and long decay time of a thermal asperity severely disrupts the stream of data samples from the digitized readback signal.
A disk drive data storage system typically includes two signal paths for the readback signal, comprising a data channel and a servo channel. When the MR head is over a customer data field, the readback signal is processed by the data channel so the system can read and write customer data to and from the disk. When the MR head is over a servo field of the disk, the readback signal is processed by the servo channel to read the servo pattern information that is pre-recorded on the disk at the time of manufacture. The MR head is typically part of a read/write head assembly in which the MR element is used for reading user data and servo pattern information from the disk, and another element (typically an inductive element) is used for writing customer data to the disk.
The read/write head assembly is mounted on a disk arm that is moved across the disk by a servo. A disk drive servo control system controls movement of the disk arm across the surface of the disk to move the read/write head assembly from data track to data track and, once over a selected track, to maintain the assembly in a path centered over the selected track. Maintaining the head assembly centered over a track facilitates accurate reading and recording of customer data. With the very high track density of current disk drives, even the smallest head positioning error can potentially cause a loss of customer data.
During data read operations in the data channel, thermal asperities can severely disrupt the stream of readback signal data samples, perhaps causing five to thirty consecutive bytes of readback data pulses to contain errors. Such a large number of consecutive errors may be difficult to correct with conventional error correction codes. Because the data channel readback signal represents customer data stored on the disk, it is imperative to completely recover all of the bytes of data that are present in the readback signal. A disk storage device is unserviceable if customer data is lost or corrupted.
In the servo channel, the readback signal produced when the transducing head is above a servo field of the disk should be approximately the same signal for each servo sector in the same track. Because the readback signal samples from the servo sectors repeat for the same track, it therefore is commonly viewed as less imperative to completely recover all of the servo information from any one servo sector field. That is, in the servo channel, any errors in the readback signal from one of the servo sectors can be replaced by values in the readback signal for the next succeeding servo sector, so each bit in the readback signal is not viewed as critically important.
In addition, a computer data storage system may typically require successfully reading from four to eight successive servo sectors before the system will declare the read/write head to be on-track so as to permit write operations. A disk drive storage system will typically require more than one revolution of the disk following a bad servo read indication for the system to realize and verify that it is actually on-track. It is possible for the successive-read requirement to give a false off-track indication, so that write operations may be inhibited even though the MR head is actually on-track.
Be cause of the critical need to recover every data pulse of the data channel signal, it is conventional to include a thermal asperity detector for the data channel. The thermal asperity detector is designed to recognize the characteristic voltage spike and long decay of the thermal asperity in the readback signal. Some relatively sophisticated processing may then be performed to recover from the induced data errors. For example, when a thermal asperity is recognized, some data storage systems adjust voltage gain levels (or keep them constant) so as to minimize the affect of the thermal asperity in the data channel processing, perhaps performing repeated read operations as well. Other data storage systems may recognize thermal asperities that repeatedly occur in the same data sector, which is then labeled as bad. Spare data sectors of the disk drive are then used to replace the data sectors declared to be bad, and customer data is moved to the spare data sector.
Conventional wisdom is that a thermal asperity detector is not needed for the servo channel, because the readback signal from servo sectors should be the same for each sector of a given disk track, and servo sectors are repeated across the surface of the disk. In fact, servo fields are repeated sufficiently (generally, approximately ninety servo sectors per disk revolution) that fully ten percent of the disk surface area is devoted to the servo fields. Thus, typically no special processing is performed in the servo channel for thermal asperities, and digitized samples from a servo sector with a thermal asperity are quickly followed by samples from successive servo sectors that are not likely affected by a thermal asperity. Therefore, corrupted readback signal samples are typically followed by uncorrupted signal samples, which it is believed should minimize the chances of commanding an incorrect head arm movement.
With demand for greater data storage capacity for disk drive systems, there is increasing demand for efficient use of disk drive resources and more precise control of disk arm movement. Accordingly, it would be advantageous if there was a reduction in the uncertainty of disk servo information and an increase in the quality and reliability of such information. For example, it would be advantageous if the disk drive controller did not have to process spurious readback signal data of servo fields if the data comes from disk sectors known to be bad, therefore permitting more processor time for tasks other than servo processing.
From the discussion above, it should be apparent that there is a need for a disk drive storage system that more efficiently utilizes storage system resources and, in particular, handles thermal asperities more efficiently. The present invention solves this need.
The present invention provides a direct access storage device (DASD) with a disk arm whose movement is controlled in response to a readback signal generated from a magneto-resistive (MR) transducer of the arm, wherein the MR transducer repeatedly passes adjacent multiple servo fields and data fields of the recording medium and receives multiple data samples, detects a thermal asperity in the readback signal when the MR transducer is above a servo field of the recording medium, determines that the servo field data samples affected by the thermal asperity are invalid due to possible data errors, and operates a servo demodulator of the DASD in response to the thermal asperity before the MR transducer is next above the servo field of the recording medium. In a DASD with a rotating disk, for example, the thermal asperity can be detected and the response can be implemented before one revolution of the disk has been completed after the thermal asperity was first encountered. This permits servo data samples that have been corrupted by the thermal asperity to be ignored, thereby removing uncertainty, and improving the quality of the servo information. Other processing operations can be undertaken in response to detection of the thermal asperity. In this way, the DASD more effectively utilizes storage system resources and, in particular, handles thermal asperities more efficiently.
In one aspect of the invention, the thermal asperity response can be implemented after the first servo field having a thermal asperity is encountered, and before another servo field is encountered. In another aspect of the invention, the servo control system can determine the location of data errors in the string of data pulses from the servo field. That is, the invention provides a pointer that permits localization of the servo sample errors as between the gray code and the servo pattern down to the individual bits in the readback signal. Alternatively, a servo sector can be determined to contain permanent thermal asperities, and readback signal samples from that sector can thereafter be ignored. If desired, the number of successful servo samples required for a read or write operation can be reduced.
Other features and advantages of the present invention should be apparent from the following description of the preferred embodiment, which illustrates, by way of example, the principles of the invention.